The invention relates to a standard for referencing luminescence signals and to a process for producing a standard of this type, and also to advantageous applications of a standard of this type.
For the purpose of this disclosure the term luminescence is understood as to include luminescence, fluorescence or both.
In addition to the desired measurement data from the analysis, the results of luminescence measurements also include device-dependent contributions which make it very difficult or virtually impossible to compare luminescence measurement data across device and laboratory boundaries and to achieve long-term comparability. For luminescence measurement data in the spectral region ranging from UV to NIR (near infrared) to be comparable, it is necessary to standardize the spectral parameters and the sensitivity parameters of luminescence measurement systems. Furthermore, the wavelength accuracy and the linearity of the detection systems typically have to be tested. Defined reference systems, such as for example luminescence standards, are required to solve this problem. The standardization of the spectral characteristics of luminescence measurement systems may take place independently of the standardization of the sensitivity parameters, which requires either luminescence intensity standards or absolute measurements of the luminescence intensity or of the luminescence quantum yield. As an alternative to physical transfer standards, such as for example receiver standards for determining the wavelength dependency of the spectral illumination intensity of the excitation channel of standard lamps or radiance standards for determining the wavelength dependency of the spectral sensitivity of the emission channel, it is also possible for chemical transfer standards, or what are known as luminescence standards, to be used for the spectral characterization of luminescence measurement systems. In this context, for the standardization of the spectral characteristics of luminescence measurement systems it is sufficient to use spectral luminescence standards with “technical” luminescence spectra corrected (for device-specific influences), given as relative or standardized luminescence intensities, attributable to the primary radiometric standard “black beam” and/or cryoradiometer.
In addition to spectral standards and intensity standards, standards which are simple to handle and have as high a long-term stability as possible are required for the characterization and testing of the wavelength accuracy, for the characterization of the day-to-day performance and for the recording of the device ageing (spectral effects and sensitivity). The demands which are imposed on standards for the referencing of luminescence signals (referred to below as “luminescence standards”) include, depending on the particular application area, inter alia                depending on the composition, luminescence in the UV to NIR spectral region,        luminescence spectra which are as unstructured and wide as possible for spectral standards,        a high and known purity,        the minimum possible overlap between absorption and emission spectra;        a wavelength-independent quantum yield of the luminescence (in the spectral region used for the device characterization),        an isotropic emission,        a low variation in the intensity at a statistically relevant number of measurement points, i.e. a high homogeneity,        a temperature dependency of the luminescence which is as low as possible and/or known in the relevant ambient temperature range,        luminescence lives in the nanosecond, microsecond or millisecond range (for lifetime standards),        as many narrow bands as possible in the UV to NIR spectral region (for wavelength standards, day-to-day performance, long-term stability, intensity standards),        a known and sufficient long-term stability (thermal and photochemical),        a high reproducibility in the case of single-use standards,        the possibility of measuring sample and transfer standard under identical measurement conditions (for example including identical measurement parameters and measurement geometry, sample formats, such as cuvette, slide, microtiter plate), at comparable signal intensities/photon counting rates, with emission characteristics that are as similar as possible.        
To make luminescence properties, which are generally measured in arbitrary and relative units, comparable, in the prior art luminescence standards are known, but in may cases these standards do not have a sufficient long-term stability, homogeneity or isotropy, or else they comprise toxic or environmentally harmful materials, such as for example cadmium or uranium.
For example, U.S. Pat. No. 4,302,678 discloses a standard for the calibration of a system which scans in the UV region and is used for the detection of surface defects on workpieces. The standard consists of a yellow potassium borosilicate glass which comprises uranium oxide. The use of uranium oxide is regarded as disadvantageous on account of the associated safety measures required and also problems of environmental protection. Furthermore, a standard of this type does not have the required photostability and long-term stability.
U.S. Pat. No. 6,770,220 discloses standards for the referencing of fluorescence signals which include sol-gel glasses, other glasses or polymers incorporating luminescent microparticles or nanoparticles. These are in particular luminescent nanoparticles of polymers and metal-ligand complexes of ruthenium, osmium, rhenium, iridium, platinum or palladium.
U.S. Pat. No. 6,123,872 discloses a luminescent glass with a long-lasting afterglow which can be used as night illumination or a night signal or as a material for confirming an infrared laser or the like. This is an oxide glass which, when excited by radiation such as gamma rays, X-rays or UV-rays, can have a long-lasting afterglow and photostimulated luminescence, the glass comprising from 1 to 55% by weight of SiO2, from 1 to 50% by weight of B2O3, from 30 to 75% by weight of ZnO, further optional constituents and terbium or manganese as fluorescent agent.
However, a glass of this type cannot be used as a luminescence standard.
A range of colored glasses which can be used as steep edge filters are known as filter glasses. These include U.S. Pat. No. 6,667,259 which discloses an optical colored glass for a steep edge filter which may comprise from 30 to 75% by weight of SiO2, 5 to 35% by weight of K2O, 0 to 5% by weight of TiO2, 4 to 7% by weight of B2O3, 5 to 30% by weight of ZnO, 0.01 to 10% by weight of F and 0.1 to 3% by weight of copper, silver, indium, gallium, aluminium, yttrium, sulphur, selenium or tellurium. This is a colored flash glass in which the coloration is produced by colloidal precipitation of semiconductor compounds during cooling of the melt or by subsequent heat treatment.
Further colored glasses of a similar type are known from U.S. patent application US 2005/0054515 A1 and from U.S. Pat. No. 4,106,946.
U.S. Pat. No. 3,773,530 discloses a further colored glass for a filter, which comprises cadmium sulphide as coloring constituent.
The photostability of colored glasses of this type is not sufficient to allow them to be used as luminescence standards.
Luminescence standards with fluorescent polymer layers on a non-fluorescent support are known from WO 02/077620 A1.
WO 01/59503 A2 discloses a luminescence standard having a substrate, for example made from quartz, to which a patterned surface of fluorescent material is applied.
DE 202004002064 U1 discloses a microarray support, which includes a substantially non-fluorescent substrate as support and at least one standard for fluorescence measurements which includes a colored glass. The colored glass comprises semiconductor compounds, which may be cadmium-semiconductor compounds or copper-, silver, indium-, gallium-, aluminium-, sulphur- or selenium-semiconductor compounds. The colored glasses comprise 30 to 75% by weight of SiO2, 5 to 35% by weight of K2O, 0 to 5% by weight of TiO2, 0.01 to 10% by weight of fluorine and 0.01 to 3% by weight of M′M′″Y″2, where M′ is Cu+ and/or Ag+, M′″ is In3+ and/or Ga3+ and/or Al3+ and Y″ is S2− and/or Se2−. The fluorescent semiconductor compounds are in the form of colloidal nanocrystals distributed through the glass.
Furthermore, however, there is a need for standards which are distinguished by a particularly high quality, i.e. in particular have a high homogeneity and isotropy, a low temperature dependency and a good long-term stability and photostability. Standards of this type could also satisfy further requirements, such as for example checking of the spectral sensitivity and wavelength accuracy. The time axis in time-resolved luminescence measurements should also be checked.
The colored glasses which are known in the prior art have proven not to satisfy these requirements, since they are not photostable. The other luminescence standards which are known in the prior art are also not of sufficient quality.